Traditional split/splitless (SSL) or programmed temperature vaporizing (PTV) injection ports for gas chromatographs typically consume large volumes of carrier gas by virtue of what is used at the split vent and septum purge vent rather than what is utilized for the actual analytical separation (column flow). To illustrate, a capillary column flow of approximately 1 standard cubic centimeter per minute (sccm) may have 50 sccm or more of split flow and 5 sccm of septum purge flow. One prior art method to reduce this consumption, e.g. “gas saver”, can reduce the split flow following an injection period. Reducing the split flow to too low a value however can result in undesirable elevated baselines. This may be caused by a continual outgassing of higher molecular weight contaminants introduced from the sample matrix, outgassing of polymeric seals such as O-rings, injection port septa and/or coring of such septa, or be caused by oxidation of the column stationary phase due to larger concentrations of oxygen which has back-diffused through the septum. Reducing these contaminants has traditionally been accomplished through dilution by using large split flows.
FIG. 1 illustrates a typical gas chromatograph inlet system of the prior art. The system includes a split/splitless (SSL) injector for injecting liquid samples. A carrier gas is delivered via an electronic pressure controller to the injector. A gas supply, e.g. helium, is introduced under pressure to a gas fitting. A fine porosity filter, e.g. a stainless steel frit, removes any particulate matter that may foul operation of the proportional valve. The proportional valve maintains a setpoint pressure within the body of the injector to establish a calculated flow in the analytical column. The proportional valve can be controlled by sensing the pressure of the injector using a pressure sensor that provides a feedback loop to the control circuit (not shown). Optionally, a chemical trap is included to scrub the carrier gas of potential contaminants, e.g. hydrocarbons and/or oxygen. Additional proportional valves allow purging and venting of some of the delivered carrier gas from the septum purge vent and split vent respectively, by calculation of the pressure drop across restrictors.
FIG. 2 illustrates a detailed example of a prior art SSL injector shown in FIG. 1. A septum, held in place by a septum nut, is pierced by the needle of a small syringe (not shown) to allow liquid to be flash vaporized within the inlet liner. The temperature of a heater block is regulated by a heater assembly (not shown). A supply of gas entering the injector assembly establishes a flow in the capillary column. There are two modes of operation: split and splitless.
In the split injection mode, a split flow is established that exits the split line. The flow exiting the split line is controlled by the electronic pressure controller of FIG. 1. This mode is used for injection of concentrated analytes to prevent overloading of the column or saturation of the detection system used at the terminal end of the column.
In the splitless mode of operation, the split line is closed during injection to cause the bulk of the sample material to be transferred to the capillary column. After a specified time interval, the split vent is opened to vent residual solvent vapors and to dilute any contaminants that might outgas from contaminated surfaces.
In both modes, far greater amounts of carrier gas are used for split flow and septum purge flow than are required for the gas chromatography (GC) column flow carrying out the analytical separation. Following a split or splitless injection, large volumes of split flow are typically maintained to dilute outgassing of residual contaminants. This results in a large consumption of high purity gas, e.g. helium.
Helium is becoming increasingly expensive and difficult to procure in some areas of the world. Helium is often the preferred gas of choice due to sensitivity, efficiency, chemical inertness, safety or other concerns. Alternate carrier gasses, e.g. hydrogen or nitrogen, can be used in some instances. For a mass spectrometer detection based system, hydrogen decreases sensitivity for electron ionization (EI) and can cause dehydrohalogenation reactions in the ion source while nitrogen can result in charge exchange reactions, and is known to be less efficient as a carrier gas.